CSCI 3136 Principles of Programming Languages Data Types and Memory - - PowerPoint PPT Presentation

CSCI 3136 Principles of Programming Languages

Data Types and Memory Management

Summer 2013 Faculty of Computer Science Dalhousie University

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What is a Type System?

A type system is a mechanism for defining types and associating them with operations that can be performed on objects of this type. A type system includes rules that specify

• Type equivalence: Do two values have the same type?
• Type compatibility: Can a value of a certain type be used in a

certain context?

• Type inference: How is the type of an expression computed from the

types of its parts?

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Types in a Language

• Strongly typed: Prohibits application of an operation to any object

not supporting this operation.

• Statically typed: Strongly typed and type checking is performed at

compile time (Pascal, C, Haskell, . . . )

• Dynamically typed:Types of operands of operations are checked at

run time (LISP, Smalltalk, . . . )

• Programmer does not specify types at all, compiler infers types from

context (e.g., ML)

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Definition of Types

Similar to subroutines in many languages, defining a type has two parts:

• A type’s declaration introduces its name into the current scope.
• A type’s definition describes the type (the simpler types it is

composed of). Three ways to think about types:

• Denotational: A type is a set of values.
• Constructive: A type is built-in or composite.
• Abstraction-based: A type is defined by an interface, the set of
• perations it supports.

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Classification of Types

Built-in types

• Integers, Booleans, characters, real numbers, . . .

Enumeration and range types (neither built-in nor composite)

• C:

enum DAY /* Defines an enumeration type */ { saturday, /* Names day and declares a */ sunday = 0, /* variable named workday with */ monday, /* that type */ tuesday, wednesday, /* wednesday is associated with 3 */ thursday, friday } workday;

• Pascal: 0..100

Composite types

• Records, arrays, files, lists, sets, pointers, . . .

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Records

• A nested record definition in Pascal:

type ore = record name : short_string; element_yielded : record name : two_chars; atomic_n : integer; atomic_weight : real; metallic : Boolean end end;

• Accessing fields
• re.element yielded.name

name of element yielded of ore

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Memory Layout of Records

Aligned (fixed ordering) Packed Aligned (optimized ordering)

− Potential waste of space + One machine operation per element access + Guaranteed layout in memory (good for systems programming) + No waste of space − Multiple machine operations per memory access + Guaranteed layout in memory (good for systems programming) ± Reduced space overhead + One machine operation per memory access − No guarantee of layout in memory (bad for systems programming)

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Contiguous Memory Layouts for 2-d Arrays

Row-major layout Column-major layout There are more sophisticated block-recursive layouts which, combined with the right algorithms, achieve much better cache efficiency than the above.

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Contiguous and Row-Pointer Memory Layout

char days[][10] = { "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday" }; days[2][3] == ’s’; char *days[] = { "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday" }; days[2][3] == ’s’;

Memory layout determines space usage and nature and efficiency of address calculations.

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Arrays

• Time at which array shape is bound is important

For example,

• static array is bound at compile time (stored in static memory)
• local array is bound at either compile time or elaboration time (i.e.,

binding creation time) (stored in stack)

• . . .
• Issues
• Memory allocation
• Bounds checks
• Index calculations (higher-dimensional arrays)

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Arrays, Lists, and Strings

A list

• Most imperative languages provide excellent built-in support for

array manipulation but not for operations on lists.

• Most functional languages provide excellent built-in support for list

manipulation but not for operations on arrays.

• Arrays are a natural way to store sequences when manipulating

individual elements in place (i.e., imperatively).

• Lists are naturally recursive and thus fit extremely well into the

recursive approach taken to most problems in functional programming.

• Strings are arrays of characters in imperative languages and lists of

characters in functional languages.

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Pointers

• Point to memory locations that store data (often of a specified type,

e.g., int*)

• Are not required in languages with reference model of variables

(Lisp, ML, CLU, Java)

• Are required for recursive types in languages with value model of

variables (C, Pascal, Ada) Storage reclamation

• Explicit (manual)
• Automatic (garbage collection)

Advantages and disadvantages of explicit reclamation + Garbage collection can incur serious run-time overhead − Potential for memory leaks − Potential for dangling pointers and segmentation faults

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Pointer Allocation and Deallocation

C

• p = (element *)malloc(sizeof(element))
• free(p)
• Explicit deallocation

Pascal

• new(p)
• dispose(p)
• Explicit deallocation

Java/C++

• p = new element() (semantics different between Java and C++,

how?)

• delete p (in C++)
• Explicit deallocation in C++, garbage collection in Java

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Dangling References

• A dangling reference is a pointer to an already reclaimed object.

Dangling references are notoriously hard to debug and a major source of program misbehaviour and security holes.

• Techniques to catch them:
• Tombstones
• Keys and locks

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Tombstones

new(p) q := p delete(p) Issues:

• Space overhead
• Runtime overhead
• Easy to change location
• f object in heap (just

change tombstone address)

• Invalid tombstones (RIP

= null pointer)

• Deallocate the

tombstones (need reference count or other garbage collection strategy)

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Locks and Keys

new(p) q := p delete(p)

• Pointer = address + key

Object has lock

• When reclaiming object, change the

lock

• Tombstones vs. locks/keys:

Efficiency comparison (space

• verhead or run-time overhead)

unclear

Most compilers do not by default generate code to check for dangling

• references. Most Pascal compilers allow the programmer to request

dynamic checks, which are usually implemented with locks and keys.

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Garbage Collection

Automatic reclamation of space/objects

• Essential for functional languages
• Popular in imperative languages (Clu, Ada, Modula-3, Java)
• Difficult to implement
• Slower than manual reclamation

Garbage collection methods

• Reference counts
• Mark and sweep
• Mark and sweep variants
• Stop and copy
• Generational technique

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Reference Counts

a = new Obj(); b = new Obj(); b = a; a = null; b = null;

• Associate reference count

with each object.

• Set to 1 when object is

allocated.

• Adjust
• when one pointer

assigned to another.

• on subroutine return.

Pros/cons + Fairly simple to implement + Fairly low cost − Does not work when there are circular references.

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Garbage

garbage root local var

• bject4
• bject1
• bject2
• bject3

static var live live live root

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Mark and Sweep

for each root variable r mark (r); sweep (); ———————————– void mark (Object p) if (!p.marked) p.marked = true; for each Object q referenced by p mark (q); ———————————– void sweep () for each Object p in the heap if (p.marked) p.marked = false else heap.release (p); ———————————–

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Mark and Sweep

Pros/cons: − More complicated to implement − Requires inspection of all allocated blocks in a sweep: costly. − High space usage if the recursion is deep. − Requires type descriptor at the beginning of each block to know the size of the block and to find the pointers in the block. + Works with circular data structures.

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